The New South Wales (NSW) premier has said it would be “absolutely unforgivable” if it emerged a huge bush fire in the Australian state was started deliberately.

On Monday morning the fire had burned 2,500 hectares after starting southwest of Sydney on Saturday, with strong winds driving the flames towards residential areas.

Investigators are working to establish how the fire started and police have declared the area where the blaze first broke out a crime scene, although the Rural Fire Service (RFS) said it was too early to say whether an arsonist was responsible.

Residents in suburbs near the bushfire including in Bangor, Illawong, Menai Pleasure Point, Sandy Point and Voyager Point worked over the weekend to protect their homes with hoses and buckets.

No one was thought to have died and no properties are believed to have been destroyed on Monday, which NSW premier Gladys Berejiklian said was a “miracle” and “a huge relief”.

Asked about the possibility the fire was started intentionally, she said: “The community would deem it absolutely unforgivable if this fire was deliberately lit.”

RFS Commissioner Shane Fitzsimmons expressed disbelief that no one had lost their homes in the fire, praising firefighters and residents for following emergency instructions.

“It is highly suspicious in the absence of any other cause like lightning,” he told Australian TV channel Network Seven.

“We don’t tolerate this sort of thing – it is irresponsible, criminal and how dare anybody if they’re involved in this put the lives of firefighters at risk and all those communities that were in the path of this fire.”

More than 500 firefighters tackled the blaze on Sunday, but the operation was scaled back to 250 firefighters overnight, he said.

The blaze was downgraded from an emergency to a “watch and act” level at 5.30pm on Sunday.